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While it certainly differs from country to country, FDA allows a discrepancy of 20%. That's significant. There have been studies that showed the labels to be off by 25%.

And even if the numbers were accurate, they're still rule-of-thumbish, because human metabolism doesn't simply ingest all energy available in a food. For instance you'd absorb roughly 40% more calories from peanut butter than from actual peanuts (of the same calorical value).

There are more reasons why counting calories isn't nearly as accurate a method as people tend to assume, but these two are really sufficient.